Many types of semiconductor devices such as dynamic random access memories (DRAMs), static rams (SRAMs), and programmable memories are formed in much the same way. Layers of oxide, nitride, and polycrystalline silicon (polysilicon or poly) are formed over a substrate such as silicon to form field and gate oxide, capacitor cell plates, word and digit lines, and various other structures.
To form a thick oxide layer such as field oxide, a thin blanket oxide layer is formed over a substrate. A blanket layer of protective material such as nitride is formed over the oxide layer, and a patterned mask is formed over the protective layer to cover regions of protective material and leaving other regions of protective material exposed. The exposed protective regions (and often the underlying oxide layer) are etched to expose the underlying oxide layer (or underlying substrate). The substrate is then oxidized to form the field oxide.
Depending on the design of the device being formed, the field oxide can often require varying distances between an adjacent field oxide, for example to form active areas within the substrate. To form the field oxide, areas of material which resist oxide formation (such as nitride) are formed which have varying distances between the areas. The distance between the nitride areas can vary depending on the design of the device. For example, the distance between a first area and a second area can be less than a distance between the second area and a third area, depending on the active areas required for a particular design of the device. One problem which can result from differing distances between structures is field oxide thinning. When the substrate is oxidized, the smaller distance between first and second areas forms a thinner field oxide than the wider area between the second and third areas. This difference in field oxide thickness can result in a device that has less desirable electrical characteristics than a device in which the field oxide thickness is uniform across the device. A process for reducing field oxide thinning in the narrow regions would be desirable.